490 THE HUMAN BODY. 



the latter through it. We shall find, however (Chap. 

 XXXII.). that light first acts upon the rod and cone layer, 

 traversing all the thickness of inner strata of the retina to 

 reach this, before it can start those changes which result in 

 visual sensations; and it is thereiore probable that the rods 

 and cones are in direct continuity with the optic nerve- 

 fibres. The limiting membranes, with the fibres of Muller 

 and their branches, are undoubtedly accessory. 



Each rod and cone consists of an outer and an inner 

 segment. The outer segments of both tend to split up trans- 

 versely into disks and are very similar, except that those of 

 the rods are longer than those of the cones and do not taper 

 as the latter do. The inner segments of the cones are 

 swollen, while those of the rods are narrow and nearly cy- 

 lindrical. Over most of the retina the rods are longer and 

 rpus than the cpnes^ but near the ciliary 



processes they cease before the cones do; and in ^le yellow 

 spot elongated cones alone are found. In this region the 

 whole retina is. much modified; at its margin all the layers 

 are thickened but espectnttythe nerve-cell layer, which is 

 here six or seven thick, while elsewhere the cells are found 

 in but one or two strata. All the fibres also are oblique, 

 reaching in to become continuous with the cones of the 

 central pit, which are long, slender and very closely packed. 

 In the fovea itself all the layers, except that of the cones, 

 thin away, and so the depression is produced. The fovea is 

 the seat of most acute vision; when we look at an object 

 we always turn our eyes so that the light proceeding from 

 it shall be focussed on this spot. Where the optic nerve 

 enters, all the layers but the nerve-fibre layer, which is very 

 thick, and the internal limiting membrane, are absent. 



The blood-vessels of the retina lie in the nerve-fibre and 

 nerve-cell layers. 



The Refracting Media of the Eye are, in succession from 

 before back, the cornea, the aqueous humor, the crystalline 

 lens, and the vitreous liumor. 



vThe_ aqueous Jiumor fills the space between the front of 

 the lens., ~2S, andIEe~5ack of the cornea. This space is in- 

 completely divided by the iris into an anterior chamber, 



